<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <script crossorigin src="https://unpkg.com/react@16/umd/react.development.js"></script>
    <script crossorigin src="https://unpkg.com/react-dom@16/umd/react-dom.development.js"></script>
    <script crossorigin src="https://unpkg.com/babel-standalone@6/babel.min.js"></script>
</head>

<body>
    <div id="app">

    </div>
    <script type="text/babel">
        class App extends React.Component{
            render(){
                return (
                    <h1>hello world</h1>
                )
            }
            componentDidMount(){
                console.log("mount")
            }
        }
        ReactDOM.render(
           <App/>,
            document.getElementById("app")
        );
    </script>
</body>

</html>